Multidimensional Charts

This work presents a new visual representation of multidimensional data and compares its usefulness in terms of effectiveness and efficiency with tabular representations. This idea makes two important contributions. First, it shows the feasibility of representing multidimensional data in 2D and 3D charts that are understandable by humans. Second, it builds on the theory of cognitive fit by testing the appropriateness of graphical representations to convey information of complex problems. In particular, it shows multidimensional data representation that has not been tested, perhaps due to the lack of a suitable graphical representation. The charts proposed are better representations of information stored in data warehouses than those provided by data cubes. The proposed format can be used to represent fuzzy variables, and are suitable for implementation in dashboards

A medium-frequency transformer with multiple secondary windings for grid connection through H-bridge voltage source converters

Although the power output of today's wind turbine has exceeded 7 MW, the voltage rating of the most common generator is below 700 V. A low-frequency transformer is commonly used to step-up the voltage to the grid voltage level, e.g. 11 kV or 33 kV. These heavy and bulky low-frequency transformers significantly increase the volume and weight of nacelle. To achieve a compact and light nacelle, a medium-voltage converter with series-connected H-bridge (SCHB) topology would be an attractive technology for future wind turbines. However, the SCHB converter requires multiple isolated and balanced DC sources, which makes the application not straightforward. As an alternative approach to generate multiple isolated and balanced sources a prototype transformer with six secondary windings, 1.26 kVA and 10 kHz, is designed and developed for 1 kV five levels SCHB multilevel converters. The experimental results show that the proposed system may be attractive in grid based renewable energy systems. © 2012 IEEJ Industry Appl Soc

Performance and cost comparison of NPC, FC and SCHB multilevel converter topologies for high-voltage applications

The rapid increase in global energy consumption and the impact of greenhouse gas emissions have accelerated the renewable energy technology into a more competitive area. Due to the variable nature of renewable energy resources and power demand by the consumers, grid based renewable generation has gained significant popularity in the world. High-voltage converter can interconnect the renewable systems to the grid directly without introducing a lossy, costly and bulky transformer. Three popular multilevel converter topologies: Neutral Point Clamped (NPC), Flying Capacitor (FC) and Series Connected H-Bridge (SCHB) have successfully made their way into the industry and therefore can be considered a mature and proven technology for low and medium voltage applications. But most of them are not suitable for high-voltage applications. This paper presents the comparison of a Five-Level (5L)-NPC, a 5L-FC, a 5L-SCHB, an Eleven-Level (11L)-NPC, an 11L-FC and an 11L-SCHB topologies for an 11 kV Voltage Source Converter (VSC). The comparison is made in terms of number of semiconductors, semiconductor cost, Total Harmonic Distortion (THD), filter size and control complexity. © 2011 IEEE

Transformer-less local grid based 11 kV SCHB multilevel converter for renewable energy systems

Due to the variable nature of renewable energy resources and power demand by the consumers, it is difficult to operate a power system installed with only one type of renewable energy resource. The local grid based renewable generation may be the only solution to overcome this problem. In this regard, an 11 kV Series Connected H-Bridge (SCHB) multilevel Voltage Source Converter (VSC) is proposed, which is the transformer less, cost effective solution to interface the renewable generation system to the local grid directly. This paper presents the design, simulation and analysis of a Five Level (5L)-SCHB and an Eleven Level (11L)-SCHB VSC for an 11 kV local grid based renewable energy systems. The performance, cost, modulation scheme and harmonic spectra of the converter are the bases for analysis. © 2011 IEEE

Steady state characteristic simulation of DFIG for wind power system

Wind power, as an important and promising renewable resource, is widely studied. Because the wind is highly variable, it is very desirable to operate a wind turbine at variable speeds. In this respect, doubly fed induction generator (DFIG) has become popular in wind power generation system. The steady state simulation analysis of a DFIG is essential to understand the behavior of DFIG such that it can operate at maximum power producing point for a given wind speed. This paper explores the steady state characteristic of a DFIG in wind power generation system using MATLAB. ©2010 IEEE

A multilevel medium-voltage inverter for step-up-transformer-less grid connection of photovoltaic power plants

Recently, medium (0.1-5 MW) and large (>5 MW) scale photovoltaic (PV) power plants have attracted great attention, where medium-voltage grid connection (typically 6-36 kV) is essential for efficient power transmission and distribution. A power frequency transformer operated at 50 or 60 Hz is generally used to step up the traditional inverter's low output voltage (usually ≤400 V) to the medium-voltage level. Because of the heavy weight and large size of the power frequency transformer, the PV inverter system can be expensive and complex for installation and maintenance. As an alternative approach to achieve a compact and lightweight direct grid connection, this paper proposes a three-phase medium-voltage PV inverter system. The 11-kV and 33-kV PV inverter systems are designed. A scaled down three-phase 1.2-kV test rig has been constructed to validate the proposed PV inverter. The experimental results are analyzed and discussed, taking into account the switching schemes and filter circuits. The experimental results demonstrate the excellent feature of the proposed PV inverter system. © 2011-2012 IEEE

FPGA-based control of modular multilevel converters: Modeling and experimental evaluation

© 2015 IEEE. In comparison with conventional two level converters, multilevel converters present lower switching losses, lower voltage stress on switching devices, lower common-mode voltages and better harmonic performance. Due to these remarkable features, nowadays the application of this technology covers a wide range, where high-quality voltages and currents are required. However, the multilevel converter requires a number of switching pulse width modulated (PWM) signals, which cannot be generated by using a single digital signal processor (DSP)/microcontroller because the available DSP at present only can provide about 12 PWM channels. In this instance, the field programmable gate array (FPGA) is the natural choice to develop the control circuit of multilevel converters. In addition, the most common software such as MATLAB/Simulink and Xilinx ISE Design Suite-based alternative design technique is used in this paper, which may reduce the developmental time and cost of the controller. The design and implementation of the switching controller, test platform, and experimental results are analyzed and discussed

Survivability of probiotics in symbiotic low fat buffalo milk yogurt

In present study, symbiotic low fat buffalo milk yogurt prototypes (plain and blueberry) were developed using a commercial starter containing probiotics. Samples were analyzed for physicochemical and microbiological properties, and the survivability of probiotics during 10 weeks of storage. Gross composition results were: total solids 11.60 ± 0.58 and 17.12 ± 0.36%, ash 0.82 ± 0.06 and 0.78 ± 0.02%, protein 4.49 ± 0.31 and 4.16 ± 0.11%, fat 0.68 ± 0.03 and 0.55 ± 0.05%, carbohydrates 5.68 ± 0.18 and 11.38 ± 0.18% for plain and blueberry flavored samples, respectively. Mineral contents (mg/g) were: Calcium 1.97 ± 0.20 and 1.72 ± 0.06, magnesium 1.63 ± 0.02 and 1.69 ± 0.01, zinc 0.07 ± 0.01 and 0.07 ± 0.00, sodium 0.87 ± 0.15 and 0.94 ± 0.12 for the plain and blueberry flavored yogurt respectively. The values of pH, titratable acidity and viscosity ranged from 4.34 to 4.01 and 4.42 to 3.70, 0.96 to 1.13% and 0.94 to 1.30%, 1.40 to 1.67 and 2.15 to 1.56 Pa.s for the plain and blueberry flavored yogurts respectively. The initial population of Bifidobacterium spp. and Lactobacillus casei were above 108 CFU/g for both the plain and blueberry flavored yogurts. Lactobacillus acidophilus was viable only for the first two weeks; however, Bifidobacterium spp and L. casei remained viable (>106 CFU/g) during the storage. The results indicate that the low fat buffalo milk yogurt is a good vehicle for developing symbiotic yogurt.Key words: Buffalo milk, symbiotic yogurt, refrigerated storage, probiotic survivability, physicochemical properties

Optimal design of high-frequency magnetic links for power converters used in grid-connected renewable energy systems

© 2014 IEEE. Recently high-frequency common magnetic links with cores of advanced magnetic materials, such as nanocrystalline and amorphous materials, have been considered as viable candidates for the development of medium-voltage power converters. This offers a new route of step-up-transformer-less compact and lightweight direct grid integration of renewable generation systems. Most importantly, it minimizes the voltage imbalance and common mode issues of the converter systems. However, the electromagnetic design of high-frequency common magnetic links is a multiphysics problem and thereby affects the system efficiency and cost. In this paper, an optimization technique is proposed and verified by prototype magnetic links. The design optimization, implementation, test platform, and experimental test results are analyzed and discussed

Design and comparison of 11 kV multilevel voltage source converters for local grid based renewable energy systems

Because the availability of renewable energy is highly variable and the power demand by the consumers could have a very different characteristic, it is very desirable to connect a renewable generation system to the grid. In this respect, the 11 kV multilevel Voltage Source Converter (VSC) has no heavy transformer and is the cost effective solution to interfacing a renewable generation system to the local grid directly. This paper presents the design and comparison of a Five-Level Neutral Point Clamped (5L-NPC), a Five-Level Flying Capacitor (5L-FC), a Five-Level Series Connected H-Bridge (5L-SCHB), an Eleven-Level Neutral Point Clamped (11L-NPC), an Eleven-Level Flying Capacitor (11L-FC), and an Eleven-Level Series Connected H-Bridge (5L-SCHB) VSC for an 11 kV local grid based converter. The cost of power semiconductors and capacitors, modulation schemes and harmonic spectra of the converters are the bases for comparison. © 2011 IEEE